If your offer, survey, or download appears spammy or is flawed in detecting the offer has been completed, or even purposely "shaves" completions off of your total count, a tactic many do, you won't make as much money and the person attempting to get the locked program or item will be more cautious the next time they're faced with a content locker. This is not a good thing, so you'll want to get an offer that converts and something that is scarce but highly sought after. It can be very difficult to find such a program or information, and many find it easier to get into scams to milk people's desperation. I find it's better to find legitimate content to lock, this way your viewers might share your page for others to complete as long as you leave the option for likes and tweets and such.